1. Field of the Invention
The present invention relates to an optical communication system using wavelength division multiplexed signal light to transmit a relatively large amount of data over an optical fiber. More particularly, the present invention relates to an optical communication system which determines the spectrum of wavelength division multiplexed signal light and controls an optical characteristic of the wavelength division multiplexed signal light or a parameter of the optical communication system in accordance with the spectrum.
2. Description of the Related Art
Multimedia networks continue to require larger capacity optical communication systems. Therefore, a significant amount of research is being performed in the area of wavelength-division multiplexing (WDM) to increase the capacity of optical communication systems.
FIG. 1 is a diagram illustrating a multiplexing portion of a conventional WDM optical communication system. Referring now to FIG. 1, a plurality of channels, CH.sub.1. . . CH.sub.N, are combined by an optical multiplexer 100 into a wavelength division multiplexed (WDM) signal light 102. An erbium doped fiber amplifier (EDFA) 104 has a wide gain band and is therefore used to amplify WDM signal light 102. An optical communication system as illustrated in FIG. 1 can be described, generally, as a transmission line medium network.
A path network, generally, is a network of various services and transmission line medium networks connected together. The individual transmission line medium networks are essentially "invisible" to individual users. Path networks typically perform functions such as multiplexing, cross-connect processing and branching/inserting. These functions usually require very high-speed signal processing. Unfortunately, a path network can easily reach an upper limit on allowable high-speed signal processing by the network. Wavelength division multiplexing may be useful to increase the high-speed signal processing of such path networks. Therefore, proposals have been set forth to increase signal processing in a path network by using wavelength division multiplexing. See, for example, K. Sato, et al, "Network Performance and . . . " IEEE, JACSC, vol. 12, No. 1, p. 159.
Moreover, an optical wave network using wavelength division multiplexing is proposed in H. Ishida, "A Transport Network with . . . ", GLOBCOM'93). Such an optical wave network is expected to improve high-speed processing, flexibility, and simplification of a network.
To realize wavelength division multiplexing in future optical wave networks, it will likely be necessary to monitor various characteristics of the WDM signal light, and to perform various control based on the monitored characteristics. For example, it may be necessary to monitor (a) the number of transmission wavelengths in the WDM signal light, (b) the spacing between wavelengths of the WDM signal light, (c) an absolute value of optical power in each channel or variations in optical power between channels in the WDM signal light, (c) a signal-to-noise ratio (SNR) in each channel in the WDM signal light, (e) a modulation factor (modulation index) in each channel or a prechirp (phase modulation component additionally superimposed in modulating a signal) in each channel of the WDM signal light, and (f) a change in spectrum of the WDM signal light due to nonlinear effect of a transmission line.
A commercially available optical spectrum analyzer can be used to monitor characteristics of WDM signal light. For example, see M. Born and E. Wolf, "Principles of Optics", 4th ed., p. 412, and "Hewlett-Packard Lightwave Test and Measurement Catalog", 1994, p. 48. In a typical optical spectrum analyzer, light is dispersed by a diffraction grating, and a light component having a specific wavelength of the dispersed light is passed through a slit. The light component passing through the slit is received by a photodetector to measure the power of the light component. A different light component having a different wavelength is obtained by rotating the diffraction grating. Accordingly, rotation of the diffraction grating and measurement of optical power are synchronously performed to allow measurement of a wavelength distribution (optical spectrum) of optical power.
However, this type of optical spectrum analyzer has a mechanical movable part for rotating or moving the diffraction grating. As a result, this type of optical spectrum analyzer lacks long-term stability and reliability. Further, this type of optical spectrum analyzer requires a time period of approximately one second for measurement of an optical spectrum, and therefore cannot provide fast enough response for many types of monitoring and control functions. Therefore, this type of optical spectrum analyzer will not allow for appropriate characteristics of WDM signal light to be monitored and controlled.
A scanning Fabry-Perot interferometer can also be used to monitor WDM signal light. See, for example, A. Yariv, "Optical Electronics", 3rd Edition, p. 92-95. However, the interferometer has a reflecting mirror which must be moved to monitor WDM signal light. As a result, the interferometer lacks long-term stability and reliability. In addition, such a device will not provide a fast enough response to allow WDM signal light providing a high-speed data transmission to be appropriately monitored and controlled.
In view of the above, conventional devices are available to determine the spectrum of an optical signal. However, these devices will not allow various characteristics of a WDM signal light or parameters of the communication system to be controlled in accordance with the spectrum.